PROJECT SUMMARY Natural products have always been at the forefront of new drug discovery, and with the advent of the post- genomic era, they will remain so well into the future. During this process, thousands of natural products have been characterized, but of these, very few have been characterized to contain the thioacid functional group. Currently, there is a critical need to discover natural products that have novel and useful scaffolds or bioactivities. Due to its extraordinary rarity, the thioacid-containing natural products therefore are mostly unexplored to date. Recently, our lab has demonstrated that the thioacid functional group is introduced by a pair of enzymes, encoded by a two-gene cassette, that catalyze the transfer of sulfur from a sulfur carrier protein to a carboxylic acid-containing natural product. This gene cassette has now been identified in over 250 gene clusters, with more than 98% uncharacterized and encoding the biosynthesis of diverse families of natural products. In order to better characterize these new thioacid natural products, and to better understand the chemistry involved, the relevant biosynthetic machinery needs further study. Due to the novelty and bioactivities of the known thioacid- containing natural products, it is expected that further exploration of this chemical space will yield new scaffolds and enzymatic activities. The long-term goal of this project is to fully characterize the thioacid moiety and its biosynthesis for a significantly improved fundamental understanding of rare sulfur-containing natural products and sulfur-incorporating enzymes. This proposal contains two aims: (i) detailed mechanistic enzymology (including kinetics, structural characterization, residue mapping, and expansion of substrate scope) of the thioacid biosynthetic machinery from the thioplatensimycin (tPTM) and thioplatencin (tPTN) pathway and (ii) the heterologous production and structural elucidation of a novel and medicinally relevant enediyne thioacid in combination with biochemical analysis of the cognate thioacid machinery. The central hypotheses of this proposal are that (i) structural characterization of the tPTM/tPTN thioacid machinery will enable improved study of other thioacid pathways and (ii) discovery and characterization of an additional thioacid-containing natural product with a novel scaffold will provide crucial information about the inclusion of the thioacid functional group in natural products. This hypothesis is supported by a bioinformatic analysis unveiling the rich diversity of thioacid-containing natural product biosynthetic gene clusters in Nature?all containing the same well-conserved thioacid biosynthesis gene cassette. The outcomes of the proposed research will be access to structures and engineered variants of sulfur-incorporating enzymes and discovery of a novel thioacid-containing enediyne natural product.